}
            // 404
            throw responseManager.new404("Thumbnail for " + form.docId + " is under generating.");
        }

        return asStream(form.docId).contentType(getImageMimeType(thumbnailFile)).stream(out -> {
            try (BufferedInputStream in = new BufferedInputStream(new FileInputStream(thumbnailFile))) {
                out.write(in);
            }
        });
    }

    /**